1. Field of the Invention
This invention relates to an improved threshold preamplifier circuit for use in a scintillation camera.
2. Description of the Prior Art
Scintillation cameras are widely used as diagnostic tools for analyzing the distribution of a radiation-emitting substance in an object under study, such as for the medical diagnosis of a human body organ. A typical scintillation camera of a type to which the present invention is applicable is a commercial version of the Anger-type scintillation camera, the basic principles of which are described in U.S. Pat. No. 3,011,057.
The scintillation camera can take a "picture" of the distribution of radioactivity throughout an object under investigation, such as an organ of the human body which has taken up a diagnostic quantity of a radioactive isotope. A scintillation camera of the Anger-type produces a picture of the radioactivity distribution by detecting individual gamma rays emitted from the distributed radioactivity in the object and passing through a collimator to produce a scintillation in a thin planar scintillation crystal. The scintillation is detected by a bank of individual photomultiplier tubes which view overlapping areas of the crystal. Appropriate electronic circuits translate the outputs of the individual photomultiplier tubes into X and Y positional coordinate signals and a Z signal which indicates generally the energy of the scintillation event and is used to determine whether the event falls within a preselected energy window. A picture of the radioactivity distribution in the object may be obtained by coupling the X and Y signals which fall within the preselected energy window to a display, such as a cathode ray oscilloscope which displays the individual scintillation events as spots positioned in accordance with the coordinate signals. The detection circuitry typically provides for integrating a large number of spots onto photographic film.
The "resolution" of a scintillation camera refers to the degree of ability of the camera faithfully to reproduce the spatial distribution of the radioactivity which is within the field of view of the device. The overall intrinsic resolution of the Anger camera detector is generally dependent on the ability of the detector to signal accurately the position coordinates of each scintillation event. There are many operations involved in the detection of each scintillation event and the signalling of its position coordinates. It has been found that the information contributed by photomultiplier tubes distant from the location of a scintillation event is substantially less accurate than that contributed by near tubes because it is based on relatively few photons arising from the scintillation event. The error or inaccuracy is compounded by the long "lever arm" associated with the distant tubes. Thus, a threshold preamplifier circuit has been developed, as described in U.S. Pat. No. 3,732,419 for improving the resolution of a scintillation camera by giving greater weight to the signals from tubes close to the location of a scintillation event than to signals from photomultiplier tubes which are more distant. This non-linear amplification scheme is accomplished by providing a threshold preamplifier circuit which has an input-output transfer characteristic such that input signals at a magnitude more than a preselected threshold magnitude produce substantially no output signal and input signals of a magnitude greater than the preselected threshold magnitude produce an amplified output signal which is substantially proportional to the magnitude of the input signal above the threshold magnitude. In conventional circuits of this type, the threshold value is selected as a constant voltage chosen as a percentage (typically one percent) of the anticipated peak of the energy for the isotope under study.
The threshold is applied to the output of each photomultiplier tube, to reduce the effect on the coordinate positioning analysis of signals received from tubes which are distant from the location of the scintillation event in the crystal. When the threshold is applied to the output of each tube, the X and Y signals change very little because the thresholding amplifiers remove a small (a light that was reflected many times) amount of signal from both sides of the axis, which cancels out in the differential summing. The Z channel after thresholding, however, which comprises the sum of all the energy signal outputs from the photomultiplier tubes can change a great deal as compared with the unthresholded Z output as the thresholding setting is changed. This causes an increase in X and Y position gain as threshold is increased. While this difference can be compensated for where a single isotope energy is being studied, difficulties arise where more than one energy source is being received. Where dual isotopes are being studied, the threshold in conventional systems is set to a single constant value as determined by the energy of the lowest energy isotope to be used. This causes the image size for the higher energy isotope incidence to be changed when used at the same time with a lower energy isotope or an isotope with multiple peaks.